Generally, in a disc brake the braking action is produced by piston-cylinder units provided in a suitable caliper and comprising a cylinder in which a piston can slide such as to be capable of acting by means of pads on opposite surfaces of a rotating disc.
Typically, the piston-cylinder unit is provided with a flexible sheathing for protecting the cylinder and piston (particularly, the sliding surfaces thereof) from the infiltration of external material and/or corrosive agents, such as dust, dirt, moisture which may prevent the piston from properly sliding within the cylinder during a braking action and/or give rise to the corrosion of the sliding surfaces of the piston and cylinder.
The flexible sheathing currently used are bellows-shaped and are applied to the piston-cylinder unit by fixing an end thereof about the piston and an opposite end about the cylinder such as to provide a protecting barrier that envelopes the piston-cylinder unit at the cylinder opening.
When the flexible protective sheathing is in a rest condition or when it is installed in a disc brake in which the piston is completely within the cylinder, such as happens for pads that are not worn up, it adopts a configuration according to which several portions thereof are wrapped on each other such as to provide one or more loops or folds. It should be noted that when the flexible sheathing is installed on a disc brake, these folds have convexities facing the pad and, particularly, facing a support plate of a friction gasket of the pad. As a result of the wear of the pad friction gaskets, the piston extends towards the disc and the flexible sheathing is elongated to adapt itself to the new position taken by the piston.
It is known that, in order to be capable of providing a disc brake with a flexible protective sheathing, a suitable seat is required to accommodate the sheathing and particularly the folds thereof when it is in a retracted condition for example in non-worn-up pad conditions in which the piston is entirely accommodated within the cylinder.
It is also known that the arrangement of this seat at the interface of cylinder, piston, pads causes an increase in the overall dimensions because, with the brake caliper dimensioning being equal, a suitable clearance is required to be left between the pads and the piston when the latter is entirely accommodated within the cylinder and also a side housing between piston and cylinder. Thereby, the total overall dimensions of the brake caliper is increased to ensure the same performance of the caliper by providing the housing for the flexible protective sheathing.
From EP 1 340 001 patent it is known to keep a suitable spacing between cylinder and pads such as to accommodate the protective sheathing in this space when the latter is retracted.
Thereby, an increase in the overall dimensions of the brake caliper is required such that this housing can be arranged within the caliper.
As it is known from U.S. Pat. No. 4,809,821 patent, a suitable annular groove can be provided proximate to the cylinder and piston head for the folds forming the flexible sheathing to be accommodated therein.
However, it is observed that this solution can weaken the resisting section of the cylinder and accordingly of the caliper body thereby reducing the stiffness and hence the performance thereof in general.
In U.S. Pat. No. 4,524,677 patent, a solution is proposed which is similar to the solution proposed by U.S. Pat. No. 4,809,821 in which the flexible sheathing is accommodated within an annular seat provided within the caliper body.
However, also in this case, the housing is provided to the detriment of material making up the caliper body thereby resulting in a reduction in the mechanical characteristics of the assembly, for example the stiffness thereof.